The present invention relates to video transmission systems and, more specifically, to a system that uses one video format to send video data defined in another format.
A typical video component complies with one of a handful of video standards, For example, conventional televisions and VCRs sold in the United States and Japan conform to the NTSC standard while those sold in most of the rest of the world conform to the PAL standard. In addition, applications such as video conferencing may use modern video standards such as the H.261 standard defined by the International Telecommunication Union (xe2x80x9cITUxe2x80x9d).
Each standard defines characteristics of the video such as the size of a frame (i.e., the size of the image displayed on the screen). The height and width of a frame typically is defined in terms of the number of rows and columns of pixels (the individual xe2x80x9cdotsxe2x80x9d of color that make up the displayed image), respectively. For example, for some applications the CIF standard specifies 352 columns and 288 rows of pixels in a frame.
Video applications such as video conferencing that send video signals between two or more terminals typically use some form of video compression. Video compression involves compressing the data that makes up the video stream so that the video data stream may be sent using less bandwidth. Thus, a compressed video signal may be sent over a channel having relatively low bandwidth or several compressed video signals may be sent over a channel that could support many fewer uncompressed video signals. As is apparent, the use of compressed video may provide significant cost savings.
To take advantage of compression and other advantages provided by the use of a particular standard, it is often necessary to send video from a video source that supports one format over a video transmission channel that supports a different format. In order to perform compression with interoperability between terminals of different vendors, several compression standards had been defined by the ITU.
Video compression standards such as ITU H.261 and H.263 do not, however, support the NTSC frame size. Instead, these standards support a frame size designated as CIF that has 288 rows and 352 columns. Some conventional systems interface NTSC video equipment with ITU-based transmission systems by scaling the NTSC frame to fit within a CIF frame. For example, an NTSC field with 240 rows is up-scaled before the video encoding process into a 288 row Image. The CIF frame (with 288 rows) is then transmitted to the receiving terminal and decoded as a CIF image. It should be emphasized, however, that this up-scaling does not provide better image resolution. Rather, there are still only 240 rows of real data.
Techniques such as these have several drawbacks. Due to the scaling operations discussed above, the encoder typically encodes an image with the full size of the transmitted format (e.g., 352xc3x97288 for CIF) instead of the true size of the original image (e.g., 320xc3x97240 for NTSC). This causes an unnecessary increase in the bit rate and in the amount of CPU processing power used to process the video (up to 30% in the NTSC-CIF example).
Moreover, these techniques scale the image twice. The encoder up-scales the original image (e.g., NTSC) as described above, then the decoder scales the received image (e.g., CIF) to conform to the size of the display or window. Since these scaling operations are not correlated, scaling distortion may occur. In view of these and other problems, a need exists for an improved method of transmitting video defined in one format using a transmission system that uses a different video format.
The invention provides a system and method for sending incoming video having a given frame size over a video transmission channel that supports a different frame size. Incoming video from a video source such as a camera is encoded by an encoder that sends video to a decoder using a video standard that supports the different frame size. The decoder processes the received video and provides it to a video destination such as a monitor.
In accordance with the Invention, when the encoder receives the incoming video, the encoder does not scale the video to fit the frame size used by the transmission system. Instead, the encoder simply encodes the data associated with the incoming video. Similarly, the encoder sends only the encoded data to the decoder. Thus, the encoder does not process or send video data associated with the areas of the transmitted frame that do not correspond to the incoming video.
The decoder decodes all of the data it receives. However, the decoded data will correspond to the original incoming frame size. This data is then scaled as necessary to fit the size of the display or the target window.
In one embodiment, the teachings of the invention are applied to embed an NTSC video stream into a CIF-based video stream. The NTSC video stream is converted to a 240xc3x97320 frame to provide a good match with the size of macro blocks and groups of macro blocks defined in the H.261 standard. In accordance with the invention, the system only encodes and sends the CIF frame macro blocks that are associated with the NTSC frame.
A system constructed according to the invention may achieve considerable improvements in compression performance in comparison to conventional systems because the system processes and sends less data yet provides the same amount of video information to the decoder. Moreover, the system may provide better image quality than some conventional systems because fewer image scaling operations may be performed on the video data.
Thus, a method of sending an incoming video stream associated with a first frame size via a transmit video stream associated with a second frame size includes the steps of associating the two frame sizes so that a portion of the pixels detained in the transmit frame is not associated with the pixels of the incoming frame, providing the transmit video stream by encoding pixels associated with the incoming video stream and not encoding pixels in the portion of the transmit frame that is not associated with the incoming frame.